In the existing wireless communication system, there exist two types of signal transmissions including: downlink transmission, where a base station sends a signal and a terminal receives and demodulates the signal; and uplink transmission, where a terminal sends a signal and a base station receives and demodulates the signal.
In the downlink transmission, in order to send data to a plurality of terminal users simultaneously in a certain period of time, a base station needs to employ such a resource allocation mode that the bandwidth and time resources of the system can be shared by the data of the plurality of users within this period of time, and the data of the plurality of users can be sent over the system bandwidth by data multiplexing in this period of time. This mode is referred to as a downlink multi-user data multiplexing mode.
In the uplink transmission, a plurality of terminal users, which lie in different areas covered by cell signals, with the distances from the terminal users to the base station being different from each other, may need to send data to the base station and communicate with the base station in a certain period of time. Thus, in this period of time, the bandwidth and time resources of the system are shared by the plurality of terminal users, and by way of resource scheduling, the bandwidth and time resources of the system are allocated to each of the users in a certain approach, so that the data of the plurality of terminal users may be sent to the base station. This mode is referred to as an uplink multiple access mode.
Generally, the downlink multi-user data multiplexing mode and the uplink multiple access mode may be called collectively as an uplink and downlink multiple access mode.
With respect to the uplink and downlink multiple access mode, three types of basic modes including Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CMDA), as well as the combination thereof, are usually employed. In the TDMA mode, the sending time period is divided into a plurality of small time slices, each of which may be separately allocated to a terminal user and other users cannot use the resource in this time slice simultaneously, that is, a single user occupies all the system bandwidth in the time slice. In the FDMA mode, system bandwidth resource is divided into a plurality of narrower frequency bands, each of which is occupied separately by a single user. In the CDMA mode, each user extends information over the whole frequency band by using a specific code sequence, a plurality of users occupy the same time and bandwidth resources in the system, and different users are separated by using different spread spectrum code sequences. In the first generation mobile communication system, the FDMA mode is employed, while in the second generation and the third generation mobile communication systems, the TDMA and CDMA modes are employed.
In the future broadband wireless communication system, with the increase of bandwidth, the multipath interference on a wireless channel will be increased significantly. If the traditional multiple access mode, such as the TDMA or CDMA mode, is still employed, the multipath interference on a signal caused by the bandwidth increase may result in serious inter-symbol interference because of the time delay spread in the wireless channel and the relatively narrow symbol width of a high-speed information stream, thereby the signal demodulation performance may be lowered. An equalization method may be employed to overcome such multipath interferences. Because of the large number of multipaths in a broadband system, and sufficient taps of a filter, sufficient training symbols, and sufficient training time are required for the employment of a traditional time domain equalization, the complexity of the equalization algorithm is increased greatly, thus the complexity of system implementation is increased, and the system performance is lowered.
To alleviate the inter-symbol interference on broadband wireless communication signals and avoid the complex equalizer, it is proposed in the industry to employ an Orthogonal Frequency Division Multiplexing (OFDM) mode to improve the performance of signal demodulation. The OFDM mode is one type of the FDMA mode. However, in the traditional FDMA technology, a frequency band is divided into a number of disjoint sub-frequency bands for transmitting data streams in parallel, and an enough guard frequency band should be reserved between two adjacent sub-channels. In an OFDM system, however, the frequency spectrums of the sub-channels are allowed to overlap with each other because of the orthogonality between sub-carriers. Therefore, compared with the conventional FDMA system, the OFDM system may, on one hand, utilize maximally the frequency spectrum resources, and on the other hand, employ a fast algorithm of Discrete Fourier Transform.
In the OFDM system, serial-to-parallel conversion is performed on a high-speed data stream, so that the duration time of continuous data symbols on each sub-carrier is increased relatively, thus the inter-symbol interference caused by time dispersion of a wireless channel may be effectively reduced, and the complexity of equalization in a receiver may be lowered. The receiver may easily process a signal through frequency domain equalization.
The generation of an OFDM signal is shown in FIG. 1. If OFDM symbols are employed, a plurality of orthogonal sub-carriers may be employed for data transmission, and therefore different users may occupy different sub-carriers, so that the multi-user multiplexing and multiple access may be implemented. However, at the same time, the OFDM multiple access mode also has disadvantages. When a pure OFDM system is applied to a cellular mobile system, if the cellular mobile system operates in a co-frequency networking mode, relatively large interference exists between the cells, because when the users of different cells send and receive data by using the same sub-carrier, the signals sent and received by the terminal users of adjacent cells may interfere with each other. Especially at the edge of a cell, a terminal of the cell is near to other cells and the signals coming from other cells are relatively strong, when the terminal receives and sends data, serious mutual interference may be generated between the signals of adjacent cells, so that the communication performance of the terminal at the cell edge is degraded drastically.
In order to avoid signal interference between adjacent cells in the case of co-frequency networking, related improved OFDM solutions are put forward. The main solution is a mode combining CDMA and OFDM. At present, there are mainly three multiple access modes combining CDMA and OFDM, which are respectively referred to as: Multi-Carrier CDMA (MC-CDMA) mode, Multi-Carrier Direct-Sequence CDMA (MC-DS-CDMA) mode, and Orthogonal Frequency Code Division Multiple Access (OFCDMA) mode with time-frequency domain two-dimensional spread spectrum and combining OFDM.
The generation of an MC-CDMA signal is shown in FIG. 2A. In the MC-CDMA shown in FIG. 2A, spread spectrum processing is first performed on each symbol in a data stream, with the length of spread spectrum code being N, and then the spread spectrum processed data is mapped to N sub-carriers modulated by the OFDM. Compared with the OFDM mode, the MC-CDMA multiple access mode is advantageous in that frequency diversity may be utilized and interference between adjacent cells of co-frequency networking may be reduced.
The generation of an MC-DS-CDMA signal is shown in FIG. 2B, and is different from the generation of the MC-CDMA signal. In the MC-DS-CDMA mode, the spread spectrum processing on each symbol is performed over each sub-carrier, that is, spread spectrum is carried out in terms of time, so that a time diversity gain may be obtained, and interference between adjacent cells of co-frequency networking may also be reduced.
Based on the above multiple access modes combining CDMA and OFDM, there also exists in the prior art the OFCDMA mode with time-frequency domain two-dimensional spread spectrum and combining OFDM.
All of the above solutions of MC-CDMA, MC-DS-CDMA, and OFDM with the time-frequency domain two-dimensional spread spectrum combine CDMA and OFDM. Through these modes combining CDMA and OFDM, a certain diversity gain and anti-multiple access interference capability can be achieved, and multi-cell co-frequency networking may be easily implemented. However, all these solutions are commonly disadvantageous in that: the allocating and scheduling of resources and the controlling of interferences are not flexible and convenient, the elimination of the multiple access interference at a receiving end costs greatly (because the information of a sending end needs to be obtained, and the receiving process is complex), and the fading and interference of a channel may result in a burst error of some symbols, and so on.